Transfer Switch Source Select Systems and Methods

ABSTRACT

According to aspects of the disclosure, a method and system are provided for transferring a load between a primary power source and a secondary power source. In accordance with the disclosure, a method of transferring a load between a first power source and a second power source includes analyzing a plurality of power sources to identify one or more power sources providing a power greater than a threshold value. The method also includes selecting a power source from the identified one or more power sources providing power greater than the threshold value. The method further includes connecting the selected power source to a transfer mechanism. The method still further includes actuating the transfer mechanism, using power provided to the transfer mechanism by the selected power source, to transfer the load from a connection with the first power source to a connection with the second power source.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Various applications require a nearly constant supply of reliableelectrical power to operate effectively. For example, hospitals mayrequire a constant and reliable supply of electricity to ensure medicalequipment in operating rooms and the like function when needed. Further,food retailers such as supermarkets and grocery stores may require aconstant and reliable supply of electricity to properly operaterefrigeration systems associated with display cases and freezers toprevent food spoilage.

While utility companies generally provide electrical power consistentlyand reliably, such power is sometimes interrupted due to inclementweather, unforeseen accidents, or maintenance. Electrical powerconsumers seeking to mitigate or avoid even minor interruptions in theirpower supply often rely on generators and other backup systems to supplyelectrical power during periods when electrical service from a utilitycompany is interrupted. Transfer switches enable these consumers toswitch between a primary electrical source (e.g., from a utilitycompany) and a secondary electrical source (e.g., a generator or otherbackup system) when one source becomes unreliable or requiresmaintenance.

Transfer switches may be manual transfer switches where, for example, anoperator throws a switch to transfer power from one source to another.Additionally or alternatively, transfer switches may be automatictransfer switches where, for example, the switch automatically senseswhen a source has lost or gained power and responsively transfers powerfrom one source to another. In one implementation, to transfer powerbetween sources, the transfer switch may initiate a control sequence inwhich the transfer switch automatically starts a standby generator andthen connects the standby generator to the load. The transfer switch mayalso automatically reconnect the utility power to the load if utilitypower is reestablished.

A bypass transfer switch may be used for applications where maintenance,inspection, and/or testing are performed while maintaining continuouspower to the load. The bypass feature typically includes a secondaryelectro-mechanical switching device (bypass switch) that can route powerto the load in a manner that circumvents a main transfer switch. Thisbypass feature allows, for example, (i) switch redundancy if a problemarises with the main transfer switch, (ii) exercising the main transferswitch without a load connection, and (iii) isolation for maintenance ofthe main transfer switch while ensuring the continuity of power to theload or loads. Any reference to transfer switch herein is meant to benon-limiting and broadly encompass any type of transfer switch.

SUMMARY

In an example, a method of transferring a load between a first powersource and a second power source includes analyzing a plurality of powersources to identify one or more power sources providing a power greaterthan a threshold value. The method also includes selecting a powersource from the identified one or more power sources providing powergreater than the threshold value. The method further includes connectingthe selected power source to a transfer mechanism. The method stillfurther includes actuating the transfer mechanism, using power providedto the transfer mechanism by the selected power source, to transfer theload from a connection with the first power source to a connection withthe second power source.

In another example, a transfer switch for selectively supplying powerfrom a plurality of power sources to an electrical load includes a powersource analyzer, a transfer mechanism, and a source select module. Thepower source analyzer is configured to analyze a plurality of powersources and determine power data based on the analysis of each powersource. The transfer mechanism is configured to selectively connect eachof the plurality of power sources to the load one power source at atime. The source select module is connected to the power source analyzerand receives the power data from the power source analyzer. The sourceselect module is configured to: (i) identify one or more of theplurality of power sources providing a power greater than a thresholdvalue, (ii) select a power source from the identified one or more powersources providing power greater than the threshold value, and (iii)cause the selected power source to be connected to the transfermechanism such that the transfer mechanism is powered by the selectedpower source when the transfer mechanism transfers a load from aconnection with a first power source of the plurality of power sourcesto a connection with a second power source of the plurality of powersources.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill become apparent by reference to the figures and the followingdetailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram representing an example transfer switchdevice.

FIG. 2 is a block diagram representing an example second stage relay forthe transfer switch of FIG. 1.

FIG. 3 is a flow chart of an example method for transferring load.

FIG. 4 is a flow chart of an example method for transferring load.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented herein. It will be readily understood that the aspects of thepresent disclosure, as generally described herein, and illustrated inthe figures, can be arranged, substituted, combined, separated, anddesigned in a wide variety of different configurations, all of which areexplicitly contemplated herein.

As used herein, the term module may refer to, be part of, or include anApplication Specific Integrated Circuit (ASIC); an electronic circuit; acombinational logic circuit; a field programmable gate array (FPGA); aprocessor (shared, dedicated, or group) that executes code; othersuitable components that provide the described functionality; or acombination of some or all of the above, such as in a system-on-chip.The term module may include memory (shared, dedicated, or group) thatstores code executed by the processor.

The term code, as used above, may include software, firmware, and/ormicrocode, and may refer to programs, routines, functions, classes,and/or objects. The term shared, as used above, means that some or allcode from multiple modules may be executed using a single (shared)processor. In addition, some or all code from multiple modules may bestored by a single (shared) memory. The term group, as used above, meansthat some or all code from a single module may be executed using a groupof processors or a group of execution engines. For example, multiplecores and/or multiple threads of a processor may be considered to beexecution engines. In various implementations, execution engines may begrouped across a processor, across multiple processors, and acrossprocessors in multiple locations, such as multiple servers in a parallelprocessing arrangement. In addition, some or all code from a singlemodule may be stored using a group of memories.

The apparatuses and methods described herein may be implemented by oneor more computer programs executed by one or more processors. Thecomputer programs include processor-executable instructions that arestored on a non-transitory tangible computer readable medium. Thecomputer programs may also include stored data. Non-limiting examples ofthe non-transitory tangible computer readable medium are nonvolatilememory, magnetic storage, and optical storage.

In line with the discussion above, one function of an automatic transferswitch is to transfer a system load from a first power source to asecond power source when the first power source experiences a poweroutage or perhaps another power fault condition (e.g., an unstable powercondition). By transferring power from the first power source to thesecond power source, the automatic transfer switch allows the load tocontinue receiving power in an uninterrupted or near constant manner.

Other functions of an automatic transfer switch include load sheddingand maintenance modes of operation. For example, if an overloadcondition occurs, it may be beneficial to shed some loads to allow forother loads to remain powered. To shed a load, the automatic transferswitch can transfer the load from an active power source (i.e., a livepower source) to an inactive power source (i.e., a dead power source).Similarly, the automatic transfer switch can transfer a load from anactive power source to an inactive power source to facilitate amaintenance mode of operation.

In general, an automatic transfer switch includes a transfer mechanism,which is operable to carry out the transfer of the load betweendifferent power sources. In many systems, the transfer mechanism itselfrequires power to operate and, thus, transfer the load between sources.As an example, the transfer mechanism can include a solenoid, whichrequires power to switch one or more contacts between differentelectrical connections to the power sources. In one approach, thetransfer mechanism is powered by the power source to which the load isbeing transferred. This approach works well in a scenario in which thepower source from which the load is being transferred is no longeractive (e.g., experiences an outage) and, thus, cannot provide enoughpower to operate the transfer mechanism.

In other scenarios, however, powering the transfer mechanism using thepower source to which the load is being transferred may not bebeneficial (or even possible). In particular, for example, in scenariosin which the load is being transferred to an inactive power source forload shedding and/or maintenance operations it may undesirable or notpossible to power the transfer mechanism with the power source to whichthe load is being transferred.

Disclosed herein is a transfer switch and method that manages powersource selection for powering a transfer mechanism. In particular, thepresent disclosure provides for a transfer switch that analyzes powersignals provided by multiple power sources to determine which powersource(s) have at least a threshold level of power for operating thetransfer mechanism. The transfer switch further selects one of the powersources based at least in part on the analysis of the power signals andthen connects the selected power source to the transfer mechanism.

In examples, the transfer switch can selectively power the transfermechanism by (i) the power source from which the load is to betransferred or (ii) the power source to which the load is to betransferred based on the analysis of power signals provided by the powersources. As such, the transfer switch of the present disclosure canadvantageously transfer a load from an inactive power source to anactive power source and/or transfer the load from an active power sourceto an inactive power source. This may beneficially facilitate, amongother things, operating the transfer switch for power continuity, loadshedding, and/or maintenance purposes.

According to additional aspects of the disclosure, the transfer switchcan protect the power sources from source to source arc-through andshort circuit conditions. In an example, the transfer switch can protectthe power sources from such conditions by employing two stages ofrelays. In an implementation, the second stage of the relays can be adouble throw double pole relay to protect against shorting the powersources. As a result, even if there is a failure that causes the firststage of relays to pass-through power from multiple power sources at onetime, the second stage relay can only connect to one of the powersources at a time. Additionally, for example, each of the first stage ofrelays can have a relatively large contact gap to inhibit or preventarc-through. The transfer switch of the present disclosure may thusfacilitate safely transferring a load between power sources using aselected one of multiple possible power sources based on operatingconditions at the time of the transfer.

FIG. 1 is a block diagram representing an example transfer switch system100 in accordance with aspects of the present disclosure. As shown inFIG. 1, the transfer switch 100 includes a plurality of power sources110, a voltage source analyzer 120, a source select module 130, atransfer request module 132, a preferred source module 134, one or morefirst stage relays 140 with controllers 142, a second stage relay 144,and a transfer mechanism 150.

In general, the power sources 110 can each provide power in the form ofan electrical signal. In particular, for example, the electrical signalcan be an alternating current (AC) voltage signal. Example power sources110 include a power utility (e.g., the electrical grid), a generator(e.g., a diesel generator or a renewable energy power generator), and/ora power storage device (e.g., a battery). Although three power sources110 are shown in FIG. 1, the transfer switch system 100 can be utilizedin connection between two and N number of power sources 110 in otherexamples.

Each power source 110 is connectable to the transfer mechanism 150 viathe second stage relay 144 and a respective one of the first stagerelays 140. The first-stage relays 140 and the second-stage relay 144are actuatable to selectively connect one of the power sources 110 tothe transfer mechanism 150, as will be described in detail below. Thetransfer mechanism 150 can include one or more electromechanicalcontactors, solid state devices, circuit breaker devices, and/or othersuitable devices for electric power transfer. In one example, thetransfer mechanism 150 includes a solenoid that causes an electricalcontact to selectively connect to one of multiple contacts, eachcorresponding to a respective one of the power sources 110. Otherexamples are also possible.

Each power source 110 is further connected to the voltage sourceanalyzer 120. The voltage source analyzer 120 senses the electricalsignals provided by the power sources 110 and outputs power dataindicative of one or more parameters sensed for the electrical signals.In an example, the power data may relate to an amplitude, a magnitude, afrequency, and/or a phase of a voltage and/or a current of theelectrical signal provided by each power source 110. The voltage sourceanalyzer 120 may thus include one or more voltage sensing circuits(e.g., including a Hall Effect sensor). The voltage source analyzer 120provides the power data to the source select module 130.

The source select module 130 also receives an input from the transferrequest module 132. In particular, the source select module 130 mayreceive a transfer request input, which indicates that the transfermechanism 150 will be actuated to transfer a load from one power source110 to another. The transfer request input may further indicate which ofthe power sources 110 is currently connected to the load and which ofthe power sources 110 will be connected to the load after the transfermechanism is actuated. In one example, the transfer request input can beprovided from the transfer request module 132 to the source selectmodule 130 responsive to an operator manually initiating the request(e.g., by throwing a switch, pushing a button, etc.). Additionally oralternatively, for example, the transfer request input can beautomatically provided by a controller (not shown) of the transferswitch system 100 responsive to the controller determining that thecurrently connected power source 110 is experiencing an issue (e.g., anoverload, an outage, etc.). In some examples, the transfer requestprovided by the transfer request module 132 may indicate a conditiondetected by the controller, which caused the controller to initiate thepower source transfer (e.g., an overload, an outage, etc.).

Responsive to the source select module 130 receiving a transfer requestfrom the transfer request module 132, the source select module 130analyzes the power data received from the voltage source analyzer 120 toselect one of the power sources 110 for powering the transfer mechanism150 during actuation of the transfer mechanism 150 (i.e., to transfer aload from one power source 110 to another). In particular, the sourceselect module 130 analyzes the power data provided by the voltage sourceanalyzer 120 to determine which of the power sources 110 has sufficientpower to operate the transfer mechanism 150. In one example, the sourceselect module 130 can do so by determining, based on the power data,which of the power sources 110 are providing a voltage signal havingpower greater than a threshold value. In an implementation, thethreshold value can be a minimum power that is required to actuate thetransfer mechanism 150. Different threshold values can be used inadditional or alternative examples. Further, in additional oralternative aspects, the source select module 130 can determine whichpower sources 110 have power greater than the threshold based on anamplitude, a magnitude, a frequency, and/or a phase indicated by thepower data (e.g., by comparing such parameter(s) to correspondingthresholds).

If only one of the power sources 110 has power greater than thethreshold value, the source select module 130 selects that power source110 and then controls the relays 140, 144 to cause the selected powersource 110 to provide power to the transfer mechanism 150 as describedbelow. On the other hand, if the source select module 130 determinesthat more than one of the power sources 110 has sufficient power toactuate the transfer mechanism 150, the source select module 130 canaccess the preferred source module 134 to retrieve preference data andthen select one of the power sources 110 based at least in part on theretrieved preference data. The preference data can thus indicate whichof the power sources 110 should be used to power the transfer mechanism150. In one example, the preference data can indicate single preferredpower source 110. In another example, the preference data can indicate ahierarchical list of power sources 110 that applies to all situations.In such an example, the source select module 130 can select the powersource 110 to use for powering the transfer mechanism 150 by determiningwhich of the power sources 110 having sufficient power (e.g., at least athreshold power) is highest on the list and then selecting that powersource 110.

In yet another example, the preference data can indicate which of thepower sources 110 to use based on one or more factors. For example, thepreference data may take into consideration the type of load that ispowered by the system, which power source 110 is currently powering theload, which power source 110 will be powering the load after thetransfer mechanism 150 is actuated, whether the transfer request wasmanually or automatically initiated, what conditions caused the transferrequest to be initiated (e.g., an overload situation versus a power losssituation), and/or metrics relating to the power data (e.g., a stabilityof the power source over a period of time).

In some implementations, the preference data can be manually input intothe preferred source module 134 via a user input device. In one example,the preferred source select 134 can include a dipswitch with number ofpositions that correspond to the number of power sources 110 connectedto the transfer device 100 and the user may indicate a preference foreach power source 110 based on the position of the dipswitchcorresponding to that power source 110. In other examples, thepreference data can be programmed into the source preference module 134by other types of input devices (e.g., a keyboard, a button pad, amouse, etc.) and/or the preference data can provided or downloaded via acommunications interface. In still other examples, the preference datacan be programmed into the source preference module 134 at the time ofmanufacture.

As noted above, the source select module 130 selects one of the powersources 110 and then causes the relays 140, 144 to connect the selectedpower source 110 to the transfer mechanism 150. For example, the sourceselect module 130 can provide control signals to controllers 142 of thefirst stage relays 140 and/or the second stage relay 144 to controlwhether the relays 140, 144 connect or disconnect the power sources 110with the transfer mechanism 150. Although the example of FIG. 1 includesrelays 140, 144 for controlling which power source 110 is connected tothe transfer mechanism 150, other types of switching devices can beutilized in other examples.

According to aspects of the present disclosure, the relays 140, 144 canbe configured to protect against arc through, short-circuits, and/orother faults. For example, the first stage relays 140 can be gap relayshaving a gap size that mitigates (or avoids) arcing at operation up to600 V. In an implementation, the gap may be approximately 3 millimeters.

Additionally, for example, the second stage relay 144 can be a doublepole double throw relay, which acts as a failsafe against shorting thepower sources 110. In this arrangement, even if there is a failure thatcauses multiple first stage relays 140 to pass through power from thepower sources 110, the second stage relay can only connect to one of thepower sources 110 at a time. This ensures that it is not physicallypossible to short the power sources 110.

The transfer switch system 100 may thus facilitate operating thetransfer mechanism 150 in a variety of scenarios. In some scenarios, thetransfer switch 100 can transfer a load from a first power source to asecond power source by providing power to the transfer mechanism 150using a power source different from the second power source (to whichthe load is being transferred). For example, the transfer switch 100 cantransfer the load from an active power source to an inactive powersource by powering the transfer mechanism 150 using the active powersource (i.e., the power source from which the load is beingtransferred). This may beneficially facilitate operating the transferswitch 100 for load shedding and/or maintenance purposes. In otherscenarios, the transfer switch 100 can transfer the load from a firstpower source to a second power source by powering the transfer mechanism150 using the second power source (i.e., the power source to which theload is being transferred).

According to some aspects of the present disclosure, the transfer switch100 may analyze the electrical signals, provide power data, and select apower source from among multiple power sources in real-time. In thisway, the transfer switch 100 may rapidly carry out a transfer of theload between power sources responsive to a transfer request.

FIG. 2 is a block diagram representing an example second stage relay 144according to an example. As shown in FIG. 2, the second stage relay 144is a double pole double switch relay. As such, the second stage relay244 includes a plurality of inputs 262A, 262B, 264A, 264B, two outputs266A, 266B, and two switches 268A, 268B for selectively switching theoutputs 266A, 266B between the inputs 262A-264B. In FIG. 2, a firstinput 262A and a second input 262B are coupled to a first power source,whereas a third input 264A and a fourth input 264B are coupled to asecond power source. The outputs 266A and 266B are coupled to thetransfer mechanism 150. As also shown in FIG. 2, the first switch 268Aselectively couples the first output 266A to the first input 264A or thethird input 266A, and the second switch 268B selectively couples thesecond output 266B to the second input 262B or the fourth input 264B. Inthis way, the switches 268A, 268B selectively switch the transfermechanism 150 between the first and power sources 110.

FIG. 3 is next a flow chart depicting an example set of operations thatcan be carried out in an implementation of a process in accordance withaspects of the present disclosure. At block 310, the method includesreceiving a transfer request. At block 312, the method includesanalyzing a plurality of power sources to identify one or more powersources providing a power greater than a threshold value. At block 314,the method includes selecting a power source from the identified one ormore power sources providing power greater than the threshold value. Atblock 316, the method includes connecting the selected power source to atransfer mechanism. At block 318, the method includes actuating thetransfer mechanism, using power provided to the transfer mechanism bythe selected powersource, to transfer the load from a connection withthe first power source to a connection with the second power source.

FIG. 4 is next a flow chart depicting an example set of operations thatcan be carried out in an implementation of a process in accordance withaspects of the present disclosure. At block 410, the method includesreceiving a transfer request. At block 412, the method includesanalyzing a plurality of power sources to identify one or more powersources providing a power greater than a threshold value. At block 414,the method includes determining that more than one of the power sourcesare providing power greater than the threshold value. At block 416, themethod includes receiving preference data. At block 418, the methodincludes selecting a power source from the identified power sourcesbased on the received preference data. The preference data indicates apreference for at least one of the plurality of power sources over atleast another of the plurality of power sources. At block 420, themethod includes connecting the selected power source to a transfermechanism. At block 422, the method includes actuating the transfermechanism, using power provided to the transfer mechanism by theselected power source, to transfer the load from a connection with thefirst power source to a connection with the second power source.

It should be understood that arrangements described herein are forpurposes of example only. As such, those skilled in the art willappreciate that other arrangements and other elements (e.g. machines,interfaces, functions, orders, and groupings of functions, etc.) can beused instead, and some elements may be omitted altogether according tothe desired results. Further, many of the elements that are describedare functional entities that may be implemented as discrete ordistributed components or in conjunction with other components, in anysuitable combination and location. Additionally, although examples aredescribed for an automatic transfer switch, the concepts describedherein can be extended to a by-pass transfer switch.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopebeing indicated by the following claims, along with the full scope ofequivalents to which such claims are entitled. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting.

What is claimed is:
 1. A method of transferring a load between a first power source and a second power source, comprising: analyzing a plurality of power sources to identify one or more power sources providing a power greater than a threshold value; selecting a power source from the identified one or more power sources providing power greater than the threshold value; connecting the selected power source to a transfer mechanism; and actuating the transfer mechanism, using power provided to the transfer mechanism by the selected power source, to transfer the load from a connection with the first power source to a connection with the second power source.
 2. The method of claim 1, wherein the first power source is an active power source providing power when connected to the load and the second power source is an inactive power source not providing power when connected to the load.
 3. The method of claim 2, wherein the selected power source is the first power source.
 4. The method of claim 1, wherein the first power source and the second power sources are both active power sources, and wherein the selected one of the power sources is the second power source.
 5. The method of claim 1, wherein the identified one or more power sources comprise more than two power sources, and wherein the selecting the power source from the identified more than two power sources is based on predetermined preference data, wherein the preference data indicates a preference for at least one of the plurality of power sources over at least another of the plurality of power sources.
 6. The method of claim 1, further wherein connecting the selected power source to the transfer mechanism comprises: actuating a first stage relay to connect the selected power source to a second stage relay; and actuating the second stage relay to connect the selected power source to the transfer mechanism via the first stage relay and the second stage relay.
 7. The method of claim 6, wherein the second stage relay is a double pole double throw switch.
 8. The method of claim 6, wherein the first stage relay has a gap of approximately 3 millimeters.
 9. The method of claim 1, wherein analyzing the plurality of power sources comprises: sensing, via a voltage sensing module, a voltage signal for each of the plurality of power sources; determining power data for each sensed voltage signal; processing the power data to determine one or more of an amplitude, a magnitude, a phase, or a frequency of each sensed voltage signal; comparing, on a per power source basis, the determined one or more of the amplitude, the magnitude, the phase, or the frequency to a threshold amplitude, a threshold phase, or a threshold frequency; and identifying the one or more power sources based on the comparison.
 10. The method of claim 9, wherein the analyzing the plurality of power sources occurs in real-time.
 11. A transfer switch for selectively supplying power from a plurality of power sources to an electrical load, the transfer switch comprising: a power source analyzer configured to analyze a plurality of power sources and determine power data based on the analysis of each power source; a transfer mechanism configured to selectively connect each of the plurality of power sources to the load one power source at a time; and a source select module connected to the power source analyzer and receiving the power data from the power source analyzer, wherein the source select module is configured to: (i) identify one or more of the plurality of power sources providing a power greater than a threshold value, (ii) select a power source from the identified one or more power sources providing power greater than the threshold value, and (iii) cause the selected power source to be connected to the transfer mechanism such that the transfer mechanism is powered by the selected power source when the transfer mechanism transfers a load from a connection with a first power source of the plurality of power sources to a connection with a second power source of the plurality of power sources.
 12. The transfer switch of claim 11, wherein the threshold value is a minimum amount of power for powering the transfer mechanism.
 13. The transfer switch of claim 11, further comprising a plurality of first stage relays for selectively connecting the plurality of power sources to the transfer mechanism.
 14. The transfer switch of claim 13, wherein each of the plurality of first stage relays is a gap relay having a gap of approximately 3 millimeters.
 15. The transfer switch of claim 13, further comprising a second stage relay, wherein each of the plurality of power sources is connectable to the transfer mechanism via the second stage relay and a respective one of the first stage relays.
 16. The transfer switch of claim 15, wherein the second stage relay is a double pole double throw switch.
 17. The transfer switch of claim 11, wherein the first power source is an active power source providing power when connected to the load and the second power source is an inactive power source not providing power when connected to the load.
 18. The transfer switch of claim 17, wherein the selected power source is the first power source.
 19. The transfer switch of claim 11, wherein the identified one or more power sources comprise two or more power sources, and wherein the selecting the power source from the identified two or more power sources is based on predetermined preference data, wherein the preference data indicates a preference for at least one of the plurality of power sources over at least another of the plurality of power sources.
 20. The transfer switch of claim 19, further comprising a preferred source module configured to receive the preference from an input device. 